Nitrous oxide (N.sub.2 O) is generated at the time of the combustion in an incineration furnace, a boiler, etc. and is discharged together with exhaust gases. Since the discharged amount of nitrous oxide is small and it does not cause acidic rain, nitrous oxide has been discharged outside without much attention as it is. However, since nitrous oxide is a stable compound having a long bench life more than 150 years, the concentration of nitrous oxide in air has been conspicuously increasing over these years. Further, recent investigations revealed that nitrous oxide is a temperature-raising gas, and causes the ozone layer to be broken. Therefore, it is important to remove nitrous oxide from the exhaust gases in the incineration furnace, etc. from the standpoint of protecting the environment.
As mentioned above, nitrous oxide is a relatively stable compound, and has low reactivity with other compounds. Therefore, nitrous oxide cannot be removed from the exhaust gases in the incineration furnace by a neutralizing agent-adding process unlike the removal of hydrogen chloride (HCl) or oxides of sulfur (SOx). Further, a divanadium pentaoxide (V.sub.2 O.sub.5 -based denitriding catalyst) to be ordinarily used for the decomposition of oxides of nitride (NOx) exhibits excellent activity with respect to nitric oxide (NO) and nitric dioxide (NO.sub.2), but does not exhibit activity to nitrous oxide (N.sub.2 O). For this reason, the conventional techniques cannot be adopted for the removal of nitrous oxide.
Under the circumstances, JP-A 60-22922 discloses a process for decomposing nitrous oxide by reduction with use of ammonia as a reducing agent, and JP-A 4-363143 discloses a process for directly decomposing nitrous oxide with use of a zeolite-based catalyst. However, since the former process needs an excess amount of ammonia, non-reacted ammonia is unfavorably discharged into air. In the latter process, since the activity of the catalyst is low, a high reaction temperature is unfavorably required. Further, since a large cost is necessary for the preparation of the catalyst in each of these processes, the running cost consequently becomes higher for exchanging the catalyst layer due to deactivation of catalyst.
Furthermore, since nitrous oxide is produced when in coexistence with NOx, SOx and water, it is preferable to remove nitrous oxide after the removal of NOx and SOx. However, since the conventional catalysts have low activity and require very high reaction temperatures, it is difficult to arrange the nitrous oxide-decomposing catalyst layer, for example, downstream the denitriding catalyst.
In addition, it is difficult to completely remove SOx in the current exhaust gas disposal system, so that even the exhaust gas having undergone the desulfurizing treatment contains a little amount of SOx. Consequently, in many zeolite-based catalysts, their structures are poisoned or broken so that the activity unfavorably drops. In this way, it is an actual situation that no satisfactory process is now available to remove nitrous oxide from the exhaust gases.